The conventional hydraulic power assist steering system comprises a hydraulic actuator for moving the steering linkage in relation to the fluid flow supplied thereto, and a rotary hydraulic control valve assembly for controlling fluid flow to the actuator in relation to the operator exerted steering torque. The control valve generally includes a cylindrical valve body rotatable within the valve housing and a spool rotatably disposed within the valve body. Hydraulic fluid is supplied to a cavity formed in the spool, and the valve body is grooved to receive fluid flow in relation to the amount of relative rotation between spool and valve body. The fluid so received is then directed to the actuator so that steering assist is developed in relation to the relative rotation of the valve body and spool.
The spool is manually rotated by the operator of the vehicle and is connected to mechanically drive the steering linkage through a lost motion coupling. A resilient element, such as a torsion bar, couples the spool and valve body to provide a centering force for aligning the spool and valve body and to permit relative rotation therebetween in relation to operator exerted steering torque, at least within the limitations of the lost motion coupling.
In systems of the type described above, the level of driver steering effort assist required to produce a given level of power assist depends primarily on the compliance of the torsion bar. If the torsion bar has relatively high compliance, a relatively low level of driver steering effort is required. This is generally desirable in low speed operation of a vehicle where relatively high steering forces are required. If the torsion bar has relatively low compliance, a relatively high level of driver steering effort is required. This is generally desirable in high speed operation of a vehicle where relatively low steering forces are required.
To overcome the engineering trade-off described above, various arrangements have been proposed for varying the driver steering effort for a given level of power assist as a function of vehicle speed. An example of one such arrangement is given in U.S. Pat. No. 4,629,025, issued to Brasier et al., Dec. 16, 1986, and assigned to the assignee of the present invention. In that arrangement, a controlled portion of the hydraulic fluid pump output is returned to the reservoir of the pump to reduce fluid flow to the steering actuator with increasing vehicle speed.